1. Technical Field
The present disclosure relates to an operational amplifier (OP AMP) and, more particularly, to a two-stage OP AMP having a class AB output stage.
2. Discussion of Related Art
A rail-to-rail OP AMP is mainly used to obtain an output voltage that swings fully from a ground voltage to a power voltage in an output buffer of a source driver to drive general electronic devices, for example, LCD (liquid crystal display) panels. Because the layout size of the rail-to-rail OP AMP is very large, however, the size of the output buffer of the source driver including a plurality of rail-to-rail OP AMPs increases as well. Thus, with the trend of decreasing the size of the source driver, there is a need to decrease the size of the OP AMP used for the output buffer of the source driver.
Although a two-stage OP AMP is used for the output buffer of the source driver, because the falling characteristic of the two-stage OP AMP is inferior, deviations are generated between signals output from the source driver. Thus, a vertical pattern or a wave pattern may be generated in the LCD panel image display due to the deviations. Also, when the two-stage OP AMP is used instead of the rail-to-rail OP AMP for the output buffer of the source driver, the falling characteristic or rising characteristic of the output voltage of the output buffer may be degraded.
Because the output stage of the rail-to-rail OP AMP is operated in class AB, the rail-to-rail OP AMP can operate the output load thereof in class AB (or push-pull). Because one side of the output stage of the two-stage OP AMP, for example, a pull-up circuit implemented by a PMOSFET, is operated as a common source amplifier while the other side, for example, a pull-down circuit implemented by an NMOSFET, is operated as a current source, it is difficult to operate the output load of the two-stage OP AMP in class AB (or push-pull). Thus, an OP AMP having a small layout size and that is capable of improving the falling characteristic or the rising characteristic is needed.